In various industrial fields such as food, machinery and chemical, a label on which letters, symbols, patterns, etc. have been printed, i.e., patterned label is stuck on products or their packaging materials to control the production process. A typical example of such a process control is a system utilizing a bar code printed label. In the bar code control system, data such as production conditions and price of products are electro-mechanically read out from the bar code label to control the production process and sales management.
However, an ordinary bar code label with adhesive obtained by applying an adhesive made of acrylic resin or the like to a film for label made of a resin or paper having a poor heat resistance is liable to be decomposed and evaporated both the film and adhesive under severe temperature conditions as high as not lower than 300.degree. C. Thus, it cannot be used in industries requiring high temperature treatment processes such as ceramics, iron industry and glass industry, e.g., process for the preparation of television cathode ray tubes including sealing and annealing steps conducted at 400.degree. to 600.degree. C. Thus, films for label and adhesives which can withstand elevated temperatures as high as not lower than 300.degree. C. have been desired.
On the other hand, heat-resistant films obtained by a process which comprises impregnating a woven cloth of long inorganic amorphous fiber, such as glass fiber and rock wool, with a heat-resistant binder resin such as silicone resin and polyamide resin, and then curing the binder have been heretofore known. Some of these heat-resistant films can withstand an elevated temperature as high as higher than 300.degree. C. only for a short period of time.
However, if such a heat-resistant label is used to prepare a label which is used in the foregoing application, the label can be attached only on products having a plain and smooth surface because the film is rigid and thus exhibits an insufficient flexibility. When the label is exposed to a high temperature while being attached on the surface of a cathode ray tube or metal plate, it is discolorated or cannot withstand the thermal expansion of the cathode ray tube or metal plate and thus suffers from cracking and peeling. Thus, the use of such a heat-resistant label at the elevated temperatures as high as higher than 300.degree. C. is limited. At elevated temperatures as high as higher than 400.degree. C., such a heat-resistant label cannot substantially be used.
In order to solve the foregoing problems, JU-A-62-142083 (The term "JU-A" as used herein means an "unexamined published Japanese utility model application") proposes a heat-resistant bar code label for the process for the production of cathode ray tubes obtained by printing a bar code on a film for label made of ceramics, enamel, metal or the like with an ink made of a glassy inorganic compound having a low melting point (glass frit), an inorganic pigment and a solvent. However, the heat-resistant bar code label thus proposed is disadvantageous in that, though being sufficiently heat resistant, it is too rigid to be stuck on the curved surface of the product. The heat-resistant bar code is also disadvantageous in that when it is exposed to elevated temperatures as high as higher than 400.degree. C. while being attached on the product with an adhesive, it falls off the product due to the thermal deterioration of the adhesive before the thermal deterioration of the label itself because the heat resistance of the adhesive is far lower than that of the label.
Under the circumstances, JP-A-1-272682 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-4-335083 (U.S. Pat. No. 5,254,644) propose a heat-resistant adhesive comprising a silicone resin. However, since such a heat-resistant adhesive can withstand the elevated temperatures as high as higher 400.degree. C. only for a short period of time, the foregoing heat-resistant bar code label will fall off the product within a short period of time when it is exposed to elevated temperatures as high as higher than 400.degree. C. while being attached on the product with such a heat-resistant adhesive. Thus, the heat resistance of the label itself can be made the best use of only by screwing the label to the product or protecting the label in a pocket on the product. The application of such a heat-resistant adhesive in the actual production process is extremely limited and other ways takes much time.
In an attempt to eliminate these difficulties and realize an automatically-applicable heat-resistant label having an excellent flexibility which doesn't deteriorate or fall off the product at elevated temperatures, W088/07937 (U.S. Pat. No. 4,971,858, EP. 308518) proposes a label comprising a film made of a resin having a high glass frit content with a bar code printed on one side thereof with a heat-resistant ink and an adhesive having a low thermal decomposition temperature applied to the other side thereof. The glass frit used in the label melts when exposed to elevated temperatures. Even after the adhesive is deteriorated or decomposed, the glass frit thus molten can cause the bar code to be fusion-bonded-to and remain on the surface of the product.
However, the glass frit used in the foregoing label a solvent-insoluble powder having a grain diameter of several .mu.ms to several scores of .mu.m. Thus, a film containing a large amount of glass frit is very brittle. Accordingly, even if such a label can be attached by means of a label sticking machine, it is often subject to breakage, causing the suspension of the production line in the worst case.
The inventors made extensive studies to accomplish the foregoing object. As a result, it was found that the use of a film comprising a specific resinous component and a specific inorganic fiber and an adhesive comprising a specific resin and a metal powder can provide a label having a satisfactory flexibility and heat resistance which exhibits an excellent external appearance and scratch resistance and doesn't fall off even after being treated at an elevated temperature. Thus, the present invention has been worked out.